This invention relates to ignitable gas generating charges for powering tools for home and industry, and more particularly is concerned with a charge for an open chamber gas powered tool, such as a fastener driving tool for driving nails, studs and the like into workpieces.
Propellant gas powered fastener driving tools are well known in the art. One class of such tools fires fasteners, such as studs, directly into a workpiece at high velocity in much the same way as a gun fires a bullet into a target. Another class of tools for driving fasteners are gas powered percussion tools. These tools have a breech mechanism for firing gas generating charges, and a plunger which is propelled through a working stroke by the gas generated by each fired charge. The driven plunger in the tool is, in turn, utilized to drive a fastener, such as a nail or stud; or to deliver an impact to a workpiece; or to perform some other work operation. Examples of gas powered percussion tools are disclosed in earlier patents of the present inventor, including U.S. Pat. Nos. 3,514,026 and 3,283,657; as well as British Pat. Nos. 1,036,224 and 1,074,195.
The present inventor is the originator of the "open chamber" type of weapon and gas percussion tool as is disclosed in his U.S. Pat. No. 3,514,026. This tool provides for a "repeater" type of action, wherein a succession of open chamber charges are successively fed into the firing chamber and each is ignited to actuate the tool for one cycle of operation. This patented tool is relatively simple to construct and operate and may be manufactured at relatively low cost, yet provides the "repeater" action that is not only labor saving over the "single shot" tools of the prior art but permits a wide variety of high production rate uses, such as driving fasteners in the building construction trades.
However, one of the most serious problems applicable to all presently available types of ignition type gas powered fastener driving tools is the relatively high cost of the gas generating charges. A conventional type of gas generating cartridge that is presently in use in conventional "closed chamber" types of tools commonly sells for about 6 cents to 8 cents for each cartridge, despite the fact that such cartridges are purchased in large quantities by the construction building trades. One of the reasons for this high cost is the cost of manufacturing the conventional types of closed end metal casings. Another, is the relatively high cost of producing the gunpowder propellant, presently selling for about $3.00 dollars per pound, whereas nitrocellulose sells for 0.77 cents per pound.
As a result of the high cost of the cartridges, ignitable types of gas generating driven tools are primarily limited for use at present to various industrial purposes only, such as for driving studs into concrete and the like, where the high cost of the cartridges is economically justified for that particular industrial application.
To reduce the high costs of processed propellant for both gun and tools uses, various kinds of less expensive propellants have been attempted in the past. For example, in the 1870's, ordinary gun cotton in natural fibrous form was used in closed cartridges for rifles. However, this material was soon abandoned because of its excessively high burning rate that produced excessive peak pressures that often ruptured the gun barrels. To reduce this too rapid burn rate, the gun cotton fibres were at a later time compressed into solid shapes and pellets, to reduce the excessive surface burning area and accordingly reduce the excessive peak burning pressures. However, this practice made it difficult to ignite the compressed material, necessitating the use of a primer explosive to break up the compressed material and ignite and then expanded material.
In a still later in time evolutionary step, the gun cotton fibres were combined with various binder materials to provide structural integrity and to permit control of the burning rates and peak burning pressures. However it was found to be difficult to obtain uniform ballistic performance by this type of binder processing, and the costs of producing this propellant was considerably increased due to the processing steps. A primer explosive was also required to break up the solidified propellant in the binder and permit ignition.
In still later in time methods of making propellant, the gun cotton, or nitrocellulose obtained from other vegetable fibres, was ground into particles of uniform size and colloidally suspended in suitable binders that were molded or extruded into shaped charges or small "grains". This process, of course, provided the virtue of insuring uniformity of the charges, and of permitting the process to yield propellant charges of controlled and uniform burning characteristics and pressures. According, variations of this manner of making propellants have been used up to and including the present time. However, in order to obtain these controlled burning characteristics and uniformity, the processing cost of producing this propellant is relatively high, and the cartridges or charges also necessitate the use of a built-in or combined primer charge for breaking apart the propellant and igniting the particles.
Thus it is seen that gun cotton, or other nitrocellulose product, in the form of relatively loose fibres or otherwise expanded, was discontinued for use as a propellant over one hundred year ago, because of its too rapid burning and too high peak pressure. Its use at later periods of time was in a more expensive to produce processed form; either colloidally suspended in a binder and molded or shaped; or compressed in the form of compacted particles with a combined primer charge; or otherwise processed in such manner as to control its too rapid rate of burning and peak pressures.
In the prior art specifically concerned with ignition type gas pressure producing charges for tools, efforts have been made to use caseless charges employing pelletized gun cotton (compressed particles) containing a primer explosive to break up the pellet and ignite the particles. However such charges were found to be relatively expensive as discused above, as well as not uniform in ignition, and to leave a residue after burning. Additionally, careless charges have been found to be extremely dangerous when stored in large quantities due to the explosive hazard in case of fire.
Additionally, efforts have been made to reduce the high cost of cartridges by employing conventional closed cylindrical cartridge cases of plastic instead of metal. However, these efforts were discontinued for the most part due to the difficulty of removing the cases after firing as a result of excessive deformation of the plastic walls.
Still further efforts have been made to use liquid propellants to power such tools. However this has been found to greatly increase the cost of the tool; requiring the provision of added mechanism for storing, metering, and igniting the liquid propellant.